Satellite communication power management system

ABSTRACT

Disclosed is a method for operating a communications signal transmitter and apparatus for carrying out the method. The method includes the steps of receiving a communications signal; sensing a signal strength of the received communications signal, the signal strength being indicative of the current user demand; adjusting an output of a power supply that supplies operating power to a communications signal transmitter amplifier in accordance with the sensed signal strength so as to increase the output of the power supply when the sensed signal strength increases and to decrease the output of the power supply when the sensed signal strength decreases; and amplifying the received communications signal with the communications signal transmitter amplifier. The step of sensing may include a step of subtracting a noise component from the received communications signal. The step of adjusting includes the steps of setting the duty cycle of a pulse width modulated signal as a function of at least the sensed signal strength; driving a switch with the pulse width modulated signal to chop a primary DC source into an AC signal; and synchronously rectifying the AC signal with an inverse of the pulse width modulated signal to provide a DC output from the power supply for supplying the operating power to the communications signal transmitter amplifier.

FIELD OF THE INVENTION

This invention relates generally to earth satellites and, in particular,to methods and apparatus for controlling transmitter power for acommunications payload of an earth satellite.

BACKGROUND OF THE INVENTION

As a constellation of communications satellites moves around the earth,individual satellites pass over regions where the population ofcommunications users may be high, such as urban centers, and also overother regions, such as oceans, deserts and mountainous areas, where thepopulation of users is significantly lower. A communication satelliteoptimized for this type of operation would require a high peak load toaverage load communication capacity, with long orbital periods at lowpower consumption. In addition, the orbits of Low Earth Orbit (LEO)satellites move with respect to the land masses, which tends todramatically vary the number of power peaks during a repeat cycle of upto several days or more.

A problem is thus created, the problem relating to efficiently providingthe high peak load to average load communication capacity.

A desirable payload for such a communications satellite includes atransponder configured for full duplex communication. The communicationspayload includes one or more such transponders having at least oneantenna to receive signals from the earth's surface, low noiseamplifier(s), frequency converter(s), amplifiers, high poweramplifier(s) and at least one transmitting antenna.

If high communication capacity were to be supplied at all times, thenthe electrical power to support the capacity would also be required atall times. In order to supply electrical power on-board such a satelliteeither a fuel-based generator, such as a nuclear reactor or a radiationheated thermopile, can be used to generate a relatively constant supplyof electrical current that is used by the communications payload. Excesspower is typically stored in batteries for eclipse usage. Alternatively,a solar array and battery system may be used.

For the solar array case power generation is disrupted periodically whenthe sun is eclipsed, during which time the batteries are partiallydischarged. In most cases the batteries supply power to dc-to-dcconverters that, in turn, supply the required voltage and requiredcurrent to the amplifiers and other active devices of thetransponder(s).

A typical synchronous orbit satellite design contemplates power systemdesigns which are sized for the maximum peak power output in both sunlitand eclipse operation. However, by using this approach the unused powercapability is wasted during periods of low power consumption.Furthermore, providing a capability to provide the maximum powercapacity at all times adds to the mass, cost, and complexity of thesatellite and, thus, increases the cost of the overall communicationssystem, including the launch vehicles.

SUMMARY OF THE INVENTION

The foregoing and other problems are overcome and the following objectsof the invention are realized by a satellite power management systemthat provides efficient delivery of communications power for variableload telecommunications traffic with non-geostationary earth orbitingsatellites.

The teaching of this invention enables the use of a lower poweredsatellite than would be required if a power capability for highcommunication capacity operation were supplied at all times. Thisenables a reduction in satellite size and mass, and a consequentreduction in satellite and launch vehicle cost. This invention furtherprovides a mechanism for detecting the communications demand inreal-time and a controller that uses this information to vary thecommunication capacity and the power drawn from the satellite batteries.

An object of this invention is thus to provide a satellite powermanagement system that provides high power for communications during atime that a satellite passes over a large population of users, andsubstantially less power when less power is required due to low or nouser demand.

A further object of this invention is to provide a satellitecommunications payload having an efficient peak to average powerconsumption ratio to minimize the cost of the overall satellitecommunications system.

Another object of this invention is to provide, in combination, anactive transmit phased array with efficient variable power amplifiers,efficient variable dc-to-dc converters, and a controller for detectingand controlling the dc-to-dc converters to provide a satellite payloadwith a high peak to average power consumption ratio.

A still further object of this invention is to provide a technique forproducing linear amplification across a wide dynamic range that resultsin proportionate power consumption.

This invention teaches a method for operating a communications signaltransmitter and apparatus for carrying out the method. The methodincludes the steps of receiving a communications signal; sensing asignal strength of the received communications signal, the signalstrength being indicative of the received power and the current userdemand; adjusting an output of a power supply that supplies operatingpower to a communications signal transmitter amplifier in accordancewith the sensed signal strength so as to increase the output of thepower supply when the sensed signal strength increases and to decreasethe output of the power supply when the sensed signal strengthdecreases; and amplifying the received communications signal with thecommunications signal transmitter amplifier. The step of sensing mayinclude a step of subtracting a noise component from the receivedcommunications signal.

The step of adjusting includes the steps of setting the duty cycle of apulse width modulated signal as a function of at least the sensed signalstrength; driving a switch with the pulse width modulated signal to chopa primary DC source into an AC signal; and synchronously rectifying theAC signal with an inverse of the pulse width modulated signal to providea DC output from the power supply for supplying the operating power tothe communications signal transmitter amplifier.

In a presently preferred embodiment of this invention the switch iscomprised of a field effect transistor (FET) that is coupled between theprimary DC power source and a synchronous rectifier. In this embodimentthe step of driving includes a step of employing a boot strap capacitorthat is coupled between the synchronously rectified AC signal and apredetermined bias potential to enhance the pulse width modulated signalthat is applied to a gate of the FET.

BRIEF DESCRIPTION OF THE DRAWINGS

The above set forth and other features of the invention are made moreapparent in the ensuing Detailed Description of the Invention when readin conjunction with the attached Drawings, wherein:

FIG. 1 is an overall block diagram of a satellite-based communicationsystem that is suitable for use in practicing this invention;

FIG. 2 is a block diagram of a transmitter amplifier power controlsystem in accordance with this invention;

FIG. 3 is a block diagram of a conventional satellite DC/DC converter;

FIG. 4 is a block diagram of a presently preferred embodiment of asatellite DC/DC converter for use in practicing this invention;

FIG. 5 is a schematic diagram of a power train portion of the satelliteDC/DC converter of FIG. 4; and

FIG. 6 is a more detailed schematic diagram of the power train portionshown in FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a generic model for the payload of a communicationssatellite 1 a of a type to which this invention pertains. Moreparticularly, FIG. 1 illustrates a typical satellite transponder 1bconfigured for full duplex communication. The communications payloadincludes one or more such transponders having a plurality of antennas 2to receive signals from the earth's surface, low noise amplifiers 3,frequency shifters or converters 4 comprised of a local oscillator and amixer, followed by amplifiers 5, high power amplifiers 6 andtransmitting antennas 7. Filters 8 are also included to pass desiredin-band signals and reject unwanted out-of-band noise signals. Onetransponder receives signals from user terminals 9a, frequency shiftsthe received user signals, and transmits the frequency shifted signalsto a ground station, such as a gateway 9b that is connected to thepublic switched telephone network (PSTN). A second transponder receivessignals from one or more of the gateways 9b, frequency shifts thereceived signals, and transmits the frequency shifted signals to theuser terminals 9b. In this manner a full duplex communication path(voice and/or data) is established between user terminals and terminalsconnected to the PSTN.

By example, the user terminals 9a (fixed or mobile) are capable ofoperating in a full duplex mode and communicate via, by example, L-bandRF links (uplink) and S-band RF links (downlink) through the return andforward satellite transponders, respectively. Uplink L-band RF links mayoperate within a frequency range of 1.61 GHz to 1.6265 GHz, bandwidth16.5 MHz, and are preferably modulated with voice signals and/or digitalsignals in accordance with a spread spectrum technique. Downlink S-bandRF links may operate within a frequency range of 2.4835 GHz to 2.5 GHz,bandwidth 16.5 MHz. The gateway 9b may communicate with the satellite lavia receive antenna 2b and transmit antenna 7a with, by example, a fullduplex C-band RF link that may operate within a range of frequenciescentered on 5 GHz. The C-band RF links bi-directionally conveycommunication feeder links, and also convey satellite commands (forwardlink) and receive telemetry information (return link). The L-band andthe S-band satellite antennas 2a and 7b, respectively, are multiple beam(preferably 16 beam) antennas that provide earth coverage within anassociated service region. The L-band and S-band satellite antennas 2aand 7b are preferably congruent with one another. As an example, a totalof approximately 3000 full duplex communications may occur through agiven one of the satellites. Two or more satellites la may each conveythe same communication between a given user terminal 9a and one of thegateways 9b by the use of spread spectrum techniques. This mode ofoperation thus provides for diversity combining at the respectivereceivers, leading to an increased resistance to fading and facilitatingthe implementation of a soft handoff procedure.

It is pointed out that all of the frequencies, bandwidths and the likethat are described herein are representative of but one particularsystem. Other frequencies and bands of frequencies may be used with nochange in the principles being discussed. As but one example, the feederlinks between the gateway 9b and the satellite la may use frequencies ina band other than the C-band, for example the Ku or Ka bands.

Reference is made to FIG. 2 for showing a block diagram of a presentlypreferred embodiment of this invention. A satellite communicationspayload 10 includes a receive antenna 12 for receiving uplinkcommunications from one or more ground-based transmitters (userterminals 9a or gateways 9b). The received signal is processed asindicated in FIG. 1 and is eventually provided to an amplifier 14 andthence to a transmit antenna 16 for transmission to one or moreground-based receivers (user terminals 9a or gateways 9b). The receiveantenna 12 and the transmit antenna 16 are preferably constructed as anarray of antenna elements.

As such, the amplifier 14 may actually represent a plurality of transmitamplifiers individual ones of which have an output coupled to an antennaelement of a phased array transmitter antenna. One suitable embodimentfor the transmit antenna 16 is described in commonly assigned U.S. Pat.No. 5,283,587, issued Feb. 1, 1994, entitled "Active Transmit PhasedArray Antenna" by Edward Hirshfield et al. The disclosure of this U.S.Patent is incorporated by reference herein in its entirety. Othersuitable embodiments for the antennas and for the overall communicationspayload are described in U.S. patent application Ser. No. 08/060,207,filed May 7, 1993, entitled "Mobile Communication Satellite Payload" byEdward Hirshfield et al. now U.S. Pat. No. 5,422,647, issued Jun. 6,1995. Reference is also made to U.S. patent application Ser. No.08/189,111, filed Jan. 31, 1994, entitled "Active Transmit Phase ArrayAntenna with Amplitude Taper" by Edward Hirshfield now U.S. Pat. No.5,504,493, issued Apr. 2, 1996.

In accordance with this invention the communications payload 10 includesa mechanism for increasing the efficiency of the communications payload10 by detecting the communications demand and employing this informationto vary the power drawn from the satellite power supply system. In thisregard there is provided a signal strength measurement block 18 havingan input coupled to an output of the receive antenna 12. The signalstrength or power at this point is indicative of the total user demandfor this particular satellite (the received power and received signalstrength are used interchangeably herein). The detected signal strengthor power is provided to a controller 20, such as a central processingunit (CPU), which generates a plurality of digital control signals 20afor controlling a DC/DC converter 22. The DC/DC converter 22 is coupledvia satellite power bus 22a (V_(IN)) to remotely connected satellitebatteries 24 and to remotely connected solar arrays 26. The batterypotential is designated as V_(BAT) while the potential generated by thesolar arrays 26 is designated as V_(SOL). The DC/DC converter 22 iscontrolled by the digital inputs 20a to vary the output voltage of theconverter 22. The output voltage of the converter 22 is designated asV_(O). The output of the DC/DC converter 22 is used to power theamplifier 14 in such a manner that a lower supply voltage is used duringa time of low user demand, while a higher supply voltage is used duringa time of high user demand. By varying the supply voltage to theamplifier 14 the power consumption of the amplifier is varied as afunction of demand. By example only, V_(O) is varied between 2 VDC and 8VDC.

In accordance with a further aspect of this invention, described indetail below, there is provided an improved DC/DC converter 22 thatovercomes a problem of a reduction in efficiency when a greater amountof voltage must be dropped across the converter. That is, when providinga reduced converter 22 output voltage the difference between V_(O) andV_(IN) must be dropped across the converter 22. In conventional DC to DCconverter designs this causes a reduction in efficiency. Any suchreduction in converter efficiency will tend to offset the efficiencyenhancement provided by operating the output amplifier 14 at a reducedpower level during periods of low user demand.

It is noted that amplifiers that draw supply power proportional to theiroutput power are well known, such as Class C or Class A-B amplifiers.These amplifiers appear to operate efficiently when the ratio of powerconsumed to signal power produced is used as the only measure. However,satellite communication systems that support multiple signalssimultaneously require that the amplifiers operate in their linearregion to produce signals with low distortion and low interferencebetween multiple signals. Class C amplifiers are not linear because theydistort, clip or limit the signals that pass through them. Class A-Bamplifiers can be made linear, but they also distort signals when thesignal power approaches the maximum rated power of the amplifier.

Alternatively, push-pull amplifiers are linear and consume powerproportional to signal power. However, at the current state-of-the-artof semiconductor devices the use of push-pull amplifiers is notpractical. This is because matched, efficient solid state devices thatcan be supplied with both negative and positive power supplies (e.g.,PNP and NPN transistors), and that are capable of operation at themicrowave frequency bands that are typically used in satellitecommunication systems, are not yet available.

In a presently preferred embodiment of this invention field effecttransistors (FETs) are used in the high power amplifier stages. FETs aregenerally linear across a wide range of primary supply voltage. Forexample, FETs are capable of operation with nearly constant gain withsupply voltages that range from 2 to 8 volts. In this case, and when 2volts is used for the supply voltage V₀, the maximum amplifier powerthat can be produced is at a minimum. Conversely, when V₀ is set to 8volts the maximum amplifier power is at a maximum. The range of maximumsignal power that can be produced in this example is (8/2)² =16 (12 dB).The power consumed by the amplifier 14 varies in a similar manner.

Thus, this invention teaches the use of signal amplifiers (preferablyFETs) that operate in a linear fashion (nominally Class A or ClassA-Class A-B) across a wide dynamic range with a power consumption thatis approximately proportional to demand.

The communications demand is preferably sensed in the low noiseamplifier portion 3 (FIG. 1) of the transponder. The signal strengthmeasurement block 18 includes a signal strength detector 18a and ananalog-to-digital (A/D) converter which sends a digital signal 18b tothe controller 20. The signal strength detector 18a may be a simplediode detector. The signal strength detector 18a can also be anoise-riding detector that outputs a signal from which a received noisecomponent is subtracted. One suitable technique to implement anoise-riding detector is to employ two diode detectors. A first diodedetector detects the out-of-band received signal and a second diodedetector, positioned after the first filter 8 or the low noise amplifier3 of FIG. 1, detects the in-band received signal. A differentialamplifier can then be employed to output a signal that represents thedifference between the out-of-band and the in-band received signals and,thus, the received power level that is a combination of all of the usersignals that are being received. The controller 20 uses the digitalreceived signal strength information to compute a value to be sent overdigital signal lines 20a to command the DC/DC converter 22 to produce aspecified supply voltage V₀ to the FET amplifier 14. The controller 20is employed to enable incorporation, via computer software, of anynuances in the control process necessary to assure operationalstability, while at the same time providing the efficiency desired.

In this regard the controller 20 may employ a linear relationshipbetween the detected received power and the corresponding transmitteroutput power. The controller may further consider the amplifier loopgain (detected input power) and the loop filtering characteristics(maximum rate of change of the detected input power) when determiningthe value of V_(O) and, hence, the output power of the transmitter 14.The controller 20 may optionally determine a value for V₀ as a functionof a predicted demand based on historical demand. Alternatively, thetime of day and geographical location can be considered to predict ananticipated user demand over a next time increment of, by example, fiveminutes. In this manner, and when approaching a large urban area duringdaylight hours, the value of V₀ can be set to provide full transmitterpower. It is also within the scope of this invention to control theoutput power level of the transmitter amplifier 14 either partially orentirely in accordance with information received over a telemetry linkfrom a ground-based controller.

As was stated previously, a further aspect of this invention is the useof a DC/DC converter to efficiently produce the variable supply voltageV₀ and the resultant prime power necessary to drive the high power FETamplifier 14. In this regard the variable voltage is generated byvarying the on-time percentage of the pulse train which is used to drivethe chopper and integrating filter within the DC/DC converter 22.

A conventional satellite DC/DC converter is illustrated in FIG. 3. A DCvoltage is provided from the satellite batteries or from the solararrays and is chopped by block 30, under the control of a variable dutyfactor multivibrator 32, into an AC signal. The AC signal is rectifiedby block 34 and filtered by block 36, to provide the output voltage. Theoutput voltage is sensed and compared to a fixed voltage reference 38 bya comparator 40. The output of the comparator 40 indicates the deviationof the output voltage from the reference voltage about a narrow range.This signal is used to control the duty factor or cycle of themultivibrator block. 32, thus providing a closed loop control system tomaintain the output voltage at a predetermined level that is specifiedby the fixed reference voltage 38.

Such converters tend to decrease in efficiency as the output voltagedecreases. This is primarily due to the use of a diode rectifier 34 torectify the chopped (AC) voltage. Such diodes have a typical voltagedrop of at least 0.6 V. When such converters are required to produce alow voltage (e.g., 2 volts), the 0.6 volt drop in the diode result in abaseline loss in efficiency of 1-2/2.6 or 23%. If, by example, the highpower amplifier 14 of FIG. 2 were to require 1000 Watts an additional230 Watts would need to be generated because of the losses in therectifying diode 34.

To avoid this wasted power, and the associated reduction in efficiency,the DC/DC converter of this invention employs power FETs (block 42, FIG.4) that are turned on synchronously, with the chopping signal, by theoutput of an inverter 32a. In FIG. 4 the signal that drives the chopperis referred to as a Pulse Width Modulated (PWM) signal, while theinverted signal that drives the synchronous rectifier 42 is referred toas a PWM* signal. Many power FETs exhibit a low electrical on-resistance(e.g., as low as several milli-ohms when several FETs are employed inparallel). The use of FETs to construct the synchronous rectifier 42thus reduces the inefficiency to less than 5% from the 23% cited abovefor the diode rectifier case.

It should be realized that the depiction of the inverter 32a forgenerating the PWM* signal is a simplification. In practice suitabledelays are inserted between the onset of either the PWM or the PWM*signals to insure that both are not simultaneously active. This preventsS1 and S2 from simultaneously conducting. Control over the timing of thePWM and PWM* signals also serves to control the conduction of theparasitic body source-drain diode within each MOSFET switch.

Also shown in FIG. 4 is the use of a digital to analog (D/A) converter44 to provide a wide range reference voltage to the comparator 40. Theinput of the D/A converter 44 is connected to the digital signal lines20a output from the controller 20 of FIG. 2.

Reference is now made to FIG. 5 for showing a simplified schematic ofthe power train portion of the DC/DC converter 22 of this invention. Theinput DC voltage (V_(IN)) is chopped by a FET designated as switch 1(S1). The gate of S1 is connected to the PWM signal. This arrangementmay be referred to as a non-isolated buck topology. In FIG. 5 aconventional buck catch diode is replaced with a power N-channel MOSFET(S2) that is driven with the PWM* signal. S2 thus functions as thesynchronous rectifier 42 of FIG. 4. The resulting DC voltage is smoothedand filtered with the inductor L and the capacitor C to provide theamplifier 14 supply voltage V₀.

FIG. 6 is a more detailed schematic that illustrates the use of a bootstrap capacitor C_(B) to generate a buck switch (S1) enhancement voltagein accordance with an aspect of this invention. In this case a biaspotential V_(B) (for example, +15 volts) is applied through an isolationdiode D to a node to which is connected C_(B) and a collector of aDarlington transistor pair comprised of T1 and T2. The base of theDarlington pair is driven with the PWM signal via a buffer B. Thebuffered PWM signal also drives the gate of an N-channel MOSFET switchS3. The emitter of the Darlington pair and the source of S3 areconnected to the gate of the buck switch S1. In that C_(B) is connectedbetween the unregulated DC potential and the bias potential V_(B), anenhanced gate drive is supplied to the buck switch S1. In practice, theconverter 22 is comprised of five interleaved stages each of which isconstructed as shown in FIG. 6.

In operation, the PWM signal being active turns on the switch S3 andalso the Darlington pair comprised of T1 and T2. The Darlington pairfunctions in a manner analogous to a pull-up resistor that is connectedbetween the source of S3 and V_(B), thereby increasing the gate driveand reducing the turn-on and turn-off times of the buck switch S1.

An advantage of this technique is an increased operating efficiencycompared with a conventional floating supply. This technique is alsocost effective as well as being amenable for use in hybrid circuits. TheDC/DC converter efficiency achieved with this circuit is greater than82% at low power and low voltage (e.g., 2 volts), and is greater than93% at maximum voltage (e.g., 8 volts) and maximum power (e.g., 1300 W).

The use of this invention, when combined with the active transmit phasedarray disclosed in U.S. Pat. No. 5,283,587, provides a significantincrease in overall satellite communications payload efficiency. This istrue at least for the reason that the operating power of the amplifiers40 and 42 in FIG. 4 of U.S. Pat. No. 5,283,587, for each element of thephased array transmit antenna, can be uniformly varied as a function ofcommunications demand.

In U.S. Pat. No. 5,283,587 there is disclosed a technique for generatinghigh efficiency multiple beams using only variable phase coefficients ina phased array antenna. By using phase only, every active element in thearray experiences the same power at the same time. As a result, theeffects of changing transmit power levels are equal in all theamplifiers. Active devices, no matter how linear their instantaneoustransfer characteristic, behave differently at different power levels.In accordance with the transmit phased array disclosed in U.S. Pat. No.5,283,587 all elements are exposed to the same signal environment at thesame time, and thus they preserve the phase information for each signalon a relative basis. This enables the antenna beam shapes to bemaintained across a wide range of operating conditions.

Although a presently preferred embodiment of this invention is wellsuited for use in individual satellites of a constellation of LEOcommunication satellites, the teaching of this invention is not limitedto only this one important application. By example, the teaching of thisinvention can be applied generally to a wide variety of transmitterapplications, both ground-based and space-based, as well as to non-LEO,such as geosynchronous, satellite systems. Furthermore, and although thecommunications signals in the preferred embodiment are in the form of aspread spectrum format; other formats, such as Time Division, MultipleAccess (TDMA) and Frequency Division, Multiple Access (FDMA), can alsobe employed.

Thus, while the invention has been particularly shown and described withrespect to a preferred embodiment thereof, it will be understood bythose skilled in the art that changes in form and details may be madetherein without departing from the scope and spirit of the invention.

What is claimed is:
 1. A method for operating a communications signaltransmitter, comprising the steps of:receiving a communications signal;sensing a signal strength of the received communications signal;adjusting an output of a power supply that supplies operating power to acommunications signal transmitter amplifier in accordance with thesensed signal strength so as to increase the output of the power supplywhen the sensed signal strength increases and to decrease the output ofthe power supply when the sensed signal strength decreases; andamplifying the received communications signal with the communicationssignal transmitter amplifier; wherein the step of adjusting includes thesteps of:setting the duty cycle of a pulse width modulated signal as afunction of at least the sensed signal strength; driving a switch meanswith the pulse width modulated signal to chop a primary DC source intoan AC signal; and synchronously rectifying the AC signal with an inverseof the pulse width modulated signal to provide a DC output from thepower supply for supplying the operating power to the communicationssignal transmitter amplifier.
 2. A method for operating a communicationspayload of a satellite, comprising the steps of:receiving communicationssignals for a plurality of users; sensing a demand for thecommunications payload at least in part from the received communicationssignals; frequency shifting the received communications signals;adjusting an output of a power supply that supplies operating power to acommunications payload signal transmitter amplifier in accordance withthe sensed demand so as to increase the output of the power supply whenthe sensed demand increases and to decrease the output of the powersupply when the sensed demand decreases; and amplifying the frequencyshifted communications signals with the communications signaltransmitter amplifier; wherein the step of adjusting includes the stepsof:setting the duty cycle of a pulse width modulated signal as afunction of at least the sensed demand; driving a first switch meanswith the pulse width modulated signal to chop a primary DC source intoan AC signal; and synchronously rectifying the AC signal with an inverseof the pulse width modulated signal to provide a DC output from thepower supply for supplying the operating power to the communicationssignal transmitter amplifier.
 3. A method as set forth in claim 2wherein the step of synchronously rectifying includes a step of drivinga second switch means with the inverse of the pulse width modulatedsignal.
 4. A method as set forth in claim 2 wherein the first switchmeans is comprised of a field effect transistor (FET) that is coupledbetween the primary DC power source and a synchronous rectifier, andwherein the step of driving includes a step of employing a boot strapcapacitor that is coupled between the synchronously rectified AC signaland a predetermined bias potential to enhance the pulse width modulatedsignal that is applied to a gate of the FET so as to reduce the turn-onand turn-off times of the FET.
 5. A communications signal transponder,comprising:a receiver for receiving a communications signal; means forsensing a signal strength of the received communications signal; atransmitter amplifier for amplifying the received communications signal;a power supply for providing operating power to the transmitteramplifier; and means for adjusting an output of the power supply inaccordance with the sensed signal strength so as to increase the outputof the power supply when the sensed signal strength increases and todecrease the output of the power supply when the sensed signal strengthdecreases; wherein said adjusting means is comprised of means forsetting the duty cycle of a pulse width modulated signal as a functionof at least the sensed signal strength; and wherein said power supply iscomprised of:means for driving a switch means with the pulse widthmodulated signal to chop a primary DC source into an AC signal; andmeans for synchronously rectifying the AC signal with an inverse of thepulse width modulated signal to provide a DC output from the powersupply for supplying the operating power to the communications signaltransmitter amplifier.
 6. A satellite communications payload,comprising:a receiver for receiving communications signals for aplurality of users; means for sensing a demand for the communicationspayload at least in part from the received communications signals; meansfor frequency shifting the received communications signals; atransmitter amplifier for amplifying the shifted communications signals;a power supply for providing operating power to the transmitteramplifier; and means for adjusting an output of said power supply inaccordance with the sensed demand so as to increase the output of thepower supply when the sensed demand increases and to decrease the outputof the power supply when the sensed demand decreases; wherein saidadjusting means is comprised of means for setting the duty cycle of apulse width modulated signal as a function of at least the senseddemand; and wherein said power supply is comprised of:means for drivinga switch means with the pulse width modulated signal to chop a primaryDC source into an AC signal; and means for synchronously rectifying theAC signal with an inverse of the pulse width modulated signal to providea DC output from the power supply for supplying the operating power tothe communications signal transmitter amplifier.
 7. A communicationssatellite payload as set forth in claim 6 wherein said means forsynchronously rectifying includes means for driving a second switchmeans with the inverse of the pulse width modulated signal.
 8. Acommunications satellite payload as set forth in claim 6 wherein thefirst switch means is comprised of a field effect transistor (FET) thatis coupled between the primary DC power source and said means forsynchronously rectifying, and wherein said driving means is comprised ofa boot strap capacitor that is coupled between the synchronouslyrectified AC signal and a predetermined bias potential to enhance thepulse width modulated signal that is applied to a gate of the FET so asto reduce the turn-on and turn-off times of the FET.
 9. A communicationspayload for use on a spacecraft, comprising:a receiver for receivinguplinked communications signals to be transmitted to a plurality ofterrestrial receivers; circuitry coupled to said receiver for sensing ademand for the communications payload at least in part from the receiveduplinked communications signals; at least one substantially lineartransmitter amplifier coupled to said receiver for amplifying thereceived communications signals prior to transmission of thecommunications signals to the terrestrial receivers; a dc--dc converterfor providing operating power to the at least one transmitter amplifier,said dc--dc converter providing an output voltage that is adjustablewithin a range of output voltages; and a controller coupled to saiddemand sensing circuitry and to said dc--dc converter for adjusting theoutput voltage of the dc--dc converter in accordance with the senseddemand so as to increase the output voltage of the dc--dc converter whenthe sensed demand increases and to decrease the output voltage of thedc--dc converter when the sensed demand decreases such that the powerconsumption of the at least one transmitter amplifier is varied as afunction of the sensed demand.
 10. A communications payload as set forthin claim 9, wherein said at least one transmitter amplifier is comprisedof a FET amplifier having substantially constant gain over said range ofoutput voltages.
 11. A communications payload as set forth in claim 10,wherein said range of output voltages is between about 2 volts and about8 volts.
 12. A communications payload as set forth in claim 10, whereinsaid FET amplifier is operated nominally Class A or Class A-Class A-B.13. A communications payload as set forth in claim 10, wherein saiddemand sensing circuitry is responsive to at least one of a signalstrength of the received communications signals, a predicted demand forthe communications payload based on historical demand, and a predicteddemand based at least in part on a position of spacecraft.
 14. Acommunications payload as set forth in claim 10, wherein said demandsensing circuitry is responsive to at least one of information generatedon the spacecraft and information generated on the ground andtransmitted to the spacecraft.